as ML detector is used, the decision boundary between two adjacent signal points will be their arithmetic mean. ∴ for s1=−3, the probability of error (p1):
P1=1 - (shaded area) for S2 : The probability of error (P2)
P2=1−( shaded area ) for S3: the probability of error P3.
P3=1−( shaded area ) for S4: The probability of error (P4)
P4=1 - (shaded area) By concluding above graph P3 i.e. probability of error when s3 is transmitted is larger among the four. ∴I=3